Scanning of large objects, e.g. large maps or technical drawings, is conventionally performed by use of a plurality of image sensors as it is a difficult and expensive task to manufacture large single image sensors. Examples of image sensors are charge coupled devices (CCD) or contact image sensors (CIS). The image sensors are typically arranged on a movable fixture, moving past a medium to be scanned, or the medium is pulled past the fixture by motor-operated rolls. The precise position of the image sensors may be determined by appropriate calibration procedures after the manufacture of a scanner. However, due to stress induced on the scanner during transportation and/or use, small movements of the image sensors on the fixture and/or small uncertainties in the motor operating the rolls or fixture may result in artefacts in the scanned images.
U.S. Pat. No. 7,298,921 discloses a document scanning method that comprises: causing relative movement between a document and first and second imaging elements, such that each of a succession of scan lines of the document is exposed in turn to the imaging elements; generating first and second image data words representative of overlapping portions of each scan line; and concatenating at least a portion of the words to generate a third word representative of the scan line, the method being characterised by the steps of cross-correlating at least a portion of each of the words to identify a portion of the second word that is included in the first word; discarding a portion of at least one of the words; concatenating the first word or the remainder thereof with the second word or the remainder thereof to form the third word; and, if necessary, compressing or expanding the third word by linear interpolation so as to obtain a word of a predetermined length. The method, however, has limited abilities to compensate for variation between image sensors, making manual calibration and/or adjustment of the positioning of the image sensors necessary.
Thus, it remains a problem to provide a versatile method for calibrating large format scanners.